Cheating with employees

I want to ask - i am working in startup i joined on 28 th of feb 2019, now its one month passed i daily asked for my offer letter they didnt provide till now,then they gave me salary on 15th of march its too late, and i am selected for an IT work and working there as a sales boy and other marketing work, so i decided to leave , i gave resignation letter in written to them on 15th march 2019 , then they refuse to accept and asked for notice period , or said not to pay my 15 days salary, firstly - notice period is not told to me at time of interview or any time before secondly- i am ready even after that , but now they ae not ready for notice period nor giving my salary thirdly- there is no documentation till now of any thing related to notice period. fortly - not giving me clarity weaather thye will pay me or not , but they bloking me from call , whatsapp not responding. i am very disturbed they just done a frod to me please help me what step i can take to get my saalary , and do i have to serve notice even if not said to me before.??